Philbin isn't a dynamic speaker, but he is very clear and moves at a measured pace. It's well worth the listen for anyone who wants a clear exposition of the legal and constitutional issues at stake.
Basically, Philbin points out:
1) The Constitution grants the power of impeachment to the House as a body--not to an individual (the Speaker) or to any committee. That means that a vote must be taken by the House as a whole, since a legislature acts by vote.
2) Since no vote authorizing an impeachment inquiry was taken by the House, purported "subpoenas" issued by the House in this matter are simply invalid. Those "subpoenas" were rightfully rejected by the President, whose duty it is to defend the constitutional powers and prerogatives of his office.
3) The refusal by the President to make his officials available goes beyond "executive privilege" and gets to the heart of the Constitution--separation of powers. As such, the president is asserting not merely a privilege but an absolute immunity from compelled testimony, as a defense against a tyrannical Legislative Branch. Philbin cites AG Janet Reno to that effect.
4) The Constitution divides the duties of the re impeachment. The House investigates and votes on impeachment, while the Senate, acting as a court, conducts a trial on the evidence presented. Pelosi is attempting to unconstitutionally bend the Senate to the will of her Dem House by demanding that the Senate conduct the investigation that the Dem House didn't see fit to conduct. The Senate must reject this attempted coup against the Constitution.